1. Field of the Invention
This invention relates to a heat-sealable plastic film with at least a two-phased structure in the thickness range up to 400 .mu.m, that is especially suitable for the tight sealing of plastic containers, for example those made of polystyrene. The plastic film is particularly suitable for sealing containers for storing foods, such as yogurt cups, for example.
2. Discussion of the Background
Instead of pots and containers made of wood or of inorganic materials such as metal, glass, or ceramic, plastic products have frequently become popular in recent times. The criteria are particularly high when the pots or containers are used to hold and preserve foods.
A critical aspect of food preservation, whether by dehydration, freezing, or sterilization, is complete prevention of microbial growth. This often makes it necessary to seal the containers gas-tight. With plastic containers, furthermore, it is necessary to consider mechanical strength, control of water content, and the effect of the atmosphere and of light (cf. Ullmann's Encyclopedia of Industrial Chemistry 5th Ed. Vol. All, 523-560; 583-618 VCH 1988; the applicable standards were also discussed there).
Aluminum covers coated with a sealing lacquer are widely used to seal plastic containers in food technology, especially for dairy products such as yogurt cups.
Such aluminum covers frequently represent a three-layered laminate whose outer layer frequently consists of (biaxially oriented) polyethylene terephthalate (0-PET), biaxially oriented polypropylene (OPP), polyamide (O-PA), or cellulose. The heat-sealable inner layer, on the other hand, usually consists of polyethylene, ethylene copolymers, or polypropylene (U.S. Pat. No. 4,753,708; G. Stehle, Neue Verpackung September 1991, pp. 94-101). However, economic and ecological factors are being brought into the discussion more and more against the use of aluminum, for example, for packaging foods, especially considering the high cost of energy in the production of aluminum.
Therefore, plastic films provided with a sealable lacquer are also coming into use instead of aluminum. The sealing lacquer matches the plastic used. Hard polyvinyl chloride (hereinafter PVC) plays a very substantial role as a relatively economical material for sealable films with good mechanical strength and good barrier characteristics, with an acrylic resin ordinarily being used as the sealing lacquer film, whose adhesive strength and melting point can be suitably modified by additives.
However the use of PVC is subject to certain objections (cf. Ullmann's Encyclopedia loc. cit. p. 593). There is therefore a trend today to replace PVC by other thermoplastic materials for disposable packages.
Plastic films produced by coextrusion are described in DE-A 3,531,036; they consist of a sealable layer of high impact strength polystyrene, a block copolymer, and a lubricant.
In EP-A 0 406,681, the problems of using heat-sealable plastic films instead of aluminum foil are properly pointed out. The substantially tighter restrictions on processing are usually conspicuous limitations. A very narrow processing range of 10.degree.-20.degree. C., which has to be held rather constant to guarantee problem-free production and satisfactory use of the sealed package, is found in most cases. This prerequisite is not always met in filling systems with several cavities for the simultaneous filling of cups.
EP-A 0 406,681 undertook the task of improving the films based on polystyrene and others that can be produced by the coextrusion process according to DE-A 3,531,036 so that the processing range and processing reliability are increased.
Satisfactory production was also said to be provided, even on filling systems with several filling cavities. In practice, this amounts to the use of higher sealing temperatures with corresponding specifications for the quality of the plastic films.
According to EP-A 0 406,681, these specifications were satisfied by a sealable plastic film produced by the coextrusion process or by lamination from two layers A and C and optionally a layer B, as well as optionally one layer each of an adhesion promoter D for joining each of the layers A, and optionally B and C, consisting of 1 to 50% of a layer of a heat-sealable, high impact strength polystyrene A, up to 95% of a support base B, and 1 to 99% of a high-melting plastic layer C, with the sum of the thicknesses or of the weight of A and optionally B and C being 100% in each case.
While polystyrene can be said to be desirable as a packaging material from the point of view of manufacturing cost and forming conditions (110.degree.-160.degree. C.), its relatively high permeability to gases and vapors can lead to problems. The difficulties caused by adhesion to the sealing heads can actually be overcome by the layered structure of the sealing film based on high impact styrene polystyrene, but this is accomplished with increased expense in production and sometimes with disposal problems.
Therefore, there still existed a need for a heat-sealable film material. In particular, the film should be suitable, if possible as a homogeneous film and without additional surface treatment, for the gas-tight sealing of containers based on polystyrene.
Above all, however, the film material should be environmentally friendly, i.e., the film material should be recyclable even when constructed of several layers.
It has now been found that the heat-sealable plastic films PF pursuant to the invention, although they belong to another class of polymers, meet the requirements of technology with regard to mechanical stability, chemical behavior, barrier properties, thermal properties, processability (see Ullmann's Encyclopedia Vol., All, loc. cit.) and recyclability, to a very great extent.